Asking for Input and Converting Types

Why this matters

Programs become useful when they respond to a person. The built-in input() pauses the program, lets the user type something, and hands it back to you.

The idea

name = input("What is your name? ")
print("Hello,", name)

The text inside input() is the prompt shown to the user. Whatever they type becomes the value.

There is one catch that trips up almost everyone: input() always gives you text (a str), even if the user types a number.

age = input("Age: ")       # the user types 30
print(age + 1)             # TypeError: can only concatenate str (not "int") to str

"30" is text, and you cannot add 1 to text. Convert it first with:

• int(...): text or float → whole number: int("30") is 30
• float(...): text or int → decimal number: float("2.5") is 2.5
• str(...): anything → text: str(30) is "30"

age = int(input("Age: "))
print(age + 1)             # works: 31

int("abc") or int("2.5") raises a ValueError. That is Python telling you the text is not a whole number.

Try it

When your program asks for input, type your answer in the Input box under the editor before pressing Run (one line for each input() call). Try the example above with age = int(input(...)), then try it with a plain input() and see the TypeError.

Your turn

Write a program that asks for the user’s age and prints how old they will be next year. If the user types 30, it must print exactly:

Next year you will be 31

Use input() and int(). Remember to type a number in the Input box before you press Run. Check will try several ages for you.

Recap

• input("prompt") reads what the user types, as text.
• Convert with int(), float() or str() before doing maths.
• Putting the conversion right around the input() is the usual pattern: int(input(...)).

Common mistakes

• Forgetting to convert: age + 1 fails with a TypeError while age is text.
• Typing nothing in the Input box (in the browser). Your program will stop with an EOFError that says so.
• Converting too early or too late. Convert once, right when you read the value.